1. Field of the Invention
The present invention relates to a power steering system for automotive vehicles, and more particularly to a construction for reducing noise generated due to pressure change of hydraulic fluid in a power steering system.
2. Description of the Prior Art
As generally known in the art, a power steering system is a system for amplifying an operating force of a driver by means of a hydraulic pump, thereby enabling the driver to easily steer an automobile even with a relatively small force.
U.S. Pat. No. 5,531,287 issued to Sherman discloses an example of such a power steering system. In the U.S. power steering system, turning of the steering wheel is converted into linear movement of a rack through an engagement between the rack and a pinion, which together constitute a steering gear. Further, a control valve operated by the rotation of the steering wheel transfers hydraulic fluid, supplied from a source of hydraulic fluid, to a cylinder selectively through a pair of fluid pressure lines. As a result, a piston linearly reciprocates in the cylinder operatively connected to the rack, so as to operate tie rods connected to both ends of the piston, thereby carrying out a steering function. The hydraulic fluid returns from the control valve to the source of hydraulic fluid through a return line.
While an automobile is driven, noise may be generated due to vibration of a steering gear according to road conditions. This noise is the so-called xe2x80x9crattle soundxe2x80x9d and is more severe especially when an automobile travels on an unpaved road at a low speed. This is generated because the hydraulic pressure in a hydraulic line system is significantly changed while opening and closing the control valve is continuously repeated during a process where an external force from the road is transferred to the steering wheel through road wheels, a steering gear, and a steering column.
In order to reduce this noise, in the U.S. power steering system, a restriction device having an orifice, whose inner diameter is smaller than an inner diameter of a rigid return tube, is integrally formed on the end of the rigid return tube connected to a flexible return tube. The end of the rigid return tube has an inner fluid channel having a substantially conical shape, whose diameter gradually decreases from the maximum diameter of the rigid return tube to the inner diameter of the orifice. Therefore, the hydraulic fluid returning to the source of hydraulic fluid through the return line undergoes changes in its pressure and speed due to the change in the cross-sectional area of the flow while the hydraulic fluid passes the orifice, thereby reducing the noise.
In a power steering system and method employing an orifice provided at a fluid channel so as to reduce noise as described above, it has been known to be preferable that the cross-sectional area of the flow changes abruptly and largely between the orifice and the fluid channel. That is, the more abruptly the cross-sectional area of the flow changes, the better a turbulent flow happens and thereby the more noise is reduced.
However, in the case of the U.S. power steering system, since the end of the rigid return tube has a conical shape, the cross-sectional area of the flow gradually changes. Therefore, the effect of reducing noise is not so large.
Further, in the U.S. power steering system, the orifice is disposed closer to a power steering reservoir in the middle of the return line. Therefore, the orifice, which is means for reducing noise, is too far from the steering gear, which is the cause of the noise, to reduce noise generated while the hydraulic fluid travels from the control valve to the orifice, thereby deteriorating the effect of noise reduction.
Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide a power steering system for automotive vehicles, which enables a cross-sectional area of the flow of hydraulic fluid returning through a return line to abruptly change, thereby significantly improving the effect of noise reduction.
It is another object of the present invention to provide a power steering system for automotive vehicles, in which an orifice for reducing noise is disposed adjacent to a source of the noise.
In order to accomplish this object, there is provided a power steering system for an automobile, the power steering system comprising: a steering gear including a pinion turned by a steering wheel and a rack engaged with the pinion; a hydraulic cylinder containing a piston operatively connected with the rack; a control valve for directing hydraulic fluid to the hydraulic cylinder selectively through first and second fluid lines; a source of hydraulic fluid for supplying pressurized hydraulic fluid; a pressure line for delivering the pressurized hydraulic fluid from the source of hydraulic fluid to the control valve; a return line for returning the hydraulic fluid from the control valve to the source of hydraulic fluid, the returning line including a first return tube made from a rigid material and a second return tube made from a flexible material, the first return tube having a first end connected with the control valve and a second end, the second return tube having a first end connected with the source of hydraulic fluid and a second end connected with the second end of the first return tube; and an orifice tube disposed in a joint portion between the first return tube and the second return tube, the orifice tube including an orifice tube body and a flange, the orifice tube body having an outer diameter equal to an inner diameter of the first return tube so that the orifice tube body can be inserted in the first return tube, the flange having an outer diameter equal to an inner diameter of the second return tube so that the flange can be inserted in the second return tube, the orifice tube having an orifice formed through the orifice tube body and the flange, the orifice having an inner diameter smaller than the inner diameters of the first and second return tubes, so as to control pressure and speed of the hydraulic fluid flowing through the orifice from the first return tube to the second return tube, thereby reducing noise.
It is preferred that the inner diameter of the first return tube has a value between 9.4 and 9.8 mm, the inner diameter of the second return tube has a value between 11.4 and 11.8 mm, and the inner diameter of the orifice has a value between 3.4 and 3.8 mm.
In accordance with another aspect of the present invention, the orifice tube may be disposed adjacent to the steering gear.